Low, degraded or contaminated transmission fluid; Faulty input or output speed sensor, or wiring and connector fault to it; Low line pressure from a clogged filter or worn pump; Faulty shift solenoid or pressure control solenoid, or valve body wear; Worn clutch packs or bands for first gear, or a failing torque converter (automatics); Worn clutch (manual or automated manual gearboxes) giving apparent ratio error; Outdated transmission control unit software where the manufacturer has a known fix.

P0731

Gear 1 Incorrect Ratio.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
7: AreaTransmission
31: FaultThe specific fault in that area

What it means

P0731 means the transmission control unit has compared input shaft speed with output shaft speed while first gear is selected and the ratio does not match the expected figure for first gear. This points to slip or an incorrect ratio in first gear. The fault is usually stored once the discrepancy has been seen for longer than a set time during a drive cycle.

Symptoms

Engine revs rise without matching acceleration in first gear (slipping)
Flare or delayed engagement when pulling away
Harsh or erratic shifts between first and second
Gearbox warning light or transmission limp mode
Poor acceleration from a standstill
Burnt smell or dark, degraded gearbox fluid

Possible causes

Low, degraded or contaminated transmission fluidlikely
Faulty input or output speed sensor, or wiring and connector fault to itlikely
Low line pressure from a clogged filter or worn pumppossible
Faulty shift solenoid or pressure control solenoid, or valve body wearpossible
Worn clutch packs or bands for first gear, or a failing torque converter (automatics)possible
Worn clutch (manual or automated manual gearboxes) giving apparent ratio errorrare
Outdated transmission control unit software where the manufacturer has a known fixrare

How to diagnose

Read all stored codes and freeze frame data, noting gear, speeds, temperature and load when the fault set
Check for other transmission, speed sensor or engine codes and deal with those first
Check fluid level, condition and smell using the correct procedure for the gearbox, as many models are sealed and need a specific temperature
Inspect speed sensor wiring, connectors and the sensor for damage, leaks or metal debris
Compare live input and output shaft speed data with wheel speed and engine speed while driving in first gear
Check live solenoid commands and gearbox temperature against the manufacturer's specification
Perform a line pressure test where the model allows it
Check for a manufacturer software update or service bulletin for the specific model and gearbox

How to fix

Correct the fluid level or replace degraded fluid and filter using the specified type
Repair damaged wiring or connectors, or replace a faulty speed sensor
Replace a faulty shift or pressure solenoid, or overhaul the valve body as the model requires
Repair or replace the pump, worn clutches or torque converter after internal inspection
Update the transmission control unit software only where a known outdated calibration is the cause
Replace or adjust the clutch assembly on manual or automated manual gearboxes if worn

Can I keep driving?

Medium severity

Short, gentle driving may be possible, but continued slipping can quickly damage clutches and overheat the gearbox, so get it checked soon. Stop driving if slip is severe, the gearbox overheats or there is a burning smell.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.